1. Field of the Invention
This application is directed to methods for the enzymatic synthesis of xcex1-sialylated oligosaccharides. Specifically, in the methods of this invention xcex12,3-sialyltransferase is employed to transfer sialic acid or an analogue thereof, employed as its CMP-nucleotide, to the non-reducing terminus of an oligosaccharide which oligosaccharide has a fucosyl group in the position penultimate to the non-reducing sugar terminus of the oligosaccharide.
2. References
The following references are cited in this application as superscript numbers at the relevant portion of the application and are incorporated herein in their entirety.
1. Horowitz, The Glycoconjugates, Vols. I-V, Pigman, Editor New York Academic Press (1977, 1978, 1982, 1983)
2. Hakomori, Adv. Cancer Res., 52:257-331 (1989)
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5. Paulson, xe2x80x9cInteraction of Animal Viruses with Cell Surface Receptorsxe2x80x9d in The Receptors Conn Ed N.Y. Acad. Press, pp. 131-219 (1985)
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31. Sialic Acids in xe2x80x9cCell Biology Monographsxe2x80x9d Schauer, Editor, Vol. 10 (1982)
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3. State of the Art
Carbohydrates and/or oligosaccharides are present on a variety of natural and pathological glycoconjugates1. Of particular interest are carbohydrates and oligosaccharides containing sialic acid residues particularly at the nonreducing sugar terminus31. Such sialic acid terminated carbohydrates and oligosaccharides are present in a number of products which have been implicated in a wide range of biological phenomena based, in part, on the concept of recognition signals carried by the carbohydrate structures and by their binding to specific ligands.
Specifically, such sialic acid terminated carbohydrates and oligosaccharides are believed to be receptors for the binding of toxins4, pathogenic agents such as viruses5, and are believed to be recognition sites for a variety of lectins, particularly those involved in cellular adhesion6,7, etc.
Similarly, certain oligosaccharides including sialic acid terminated oligosaccharides have been identified as capable of suppressing a cell-mediated immune response to an antigen. The ability of such oligosaccharides to suppress a cell mediated immune response to an antigen is described by Venot et al.3 
Additionally, the presence of certain sialyl terminated oligosaccharides in tumor-related antigens is documented in the art1 and, in general, the structures of the oligosaccharides present on such antigens have been modified in some way from normal oligosaccharides so as to lead to the expression of tumor related antigens2. The prospect of passive immunotherapy with monoclonal antibodies directed against some sialylated tumor-associated antigens, such as the gangliosides GD2, GD3 and GM2, in patients with melanoma has been investigated8,9.
The synthesis of such oligosaccharides often involves complex chemical reactions with corresponding low yields. Accordingly, there has been much interest in using glycosyltransferases in synthesizing at least a part of these molecules.
Glycosyltransferases are a highly polymorphic group of membrane-bound enzymes of endoplasmic reticulum and Golgi bodies that catalyze the transfer of a single monosaccharide unit from a nucleotide donor to the hydroxyl group of an acceptor saccharide in the biosynthesis of N-glycan (Asn-GlcNAc N-glycosidic linkage; GlcNAc, N-acetylglucosamine) and O-glycan (Ser/Thr-GalNAc, O-glycosidic linkage; GalNAc, N-acetylgalactosamine) moieties of glycoproteins and glycolipids.
The eukaryotic sialyltransferases comprise a family of glycosyltransferases that catalyze the transfer of N-acetylneuraminic acid (NeuAc), a sialic acid (SA), from CMP-SA to the non-reducing terminus of oligosaccharide chains of glycoconjugates. The addition of the sialic acid normally terminates oligosaccharide chain elongation except for polysialic chains found on neural cell adhesion molecule and gangliosides.
Known eukaryotic sialyltransferases involved in the synthesis of N- and O-glycan derivatives of the glycoprotien and glycolipid are summarized in Table 1, dapted from Palcic63. In the table, the R represents the remainder of the acceptor glycoprotein, glycolipid or oligosaccharide chain.
xcex12,3-sialyltransferases are useful eukaryotic enzymes for in vitro synthesis of N-linked and O-linked sialyl derivatives of glycoproteins, for determinations of acceptors, and other qualitative and quantitative research of glycoproteins. However, it was previously reported that 2,3-sialyltransferases would not synthesize N-linked and O-linked sialyl derivatives of glycoproteins or glycolipids where the acceptor glycoprotein or glycolipid possessed a fucosyl derivative in the penultimate position to the non-reducing sugar terminus of the oligosaccharide (U.S. Pat. No. 5,374,65567). This necessitated careful planning in the synthesis of certain fucosylated and sialylated oligosaccharides and in some cases required that certain steps be completed using chemical synthesis, rather than enzymatic synthesis.
In view of the above, it would be particularly advantageous to develop methods for the facile preparation of xcex1-sialylated oligosaccharides from oligosaccharides having a fucosyl derivative in the penultimate position to the non-reducing sugar terminus of the oligosaccharide. The present invention accomplishes this by using an xcex12,3-sialyltransferase to effect efficient coupling of sialic acid activated as its CMP-nucleotide derivative (a donor saccharide) to a saccharide or an oligosaccharide having a fucosyl derivative in the penultimate position of the non-reducing end of the sugar moiety (acceptor oligosaccharide).
The present invention is directed to methods for the synthesis of oligosaccharides, glycoproteins and glycolipids terminated in the non-reducing sugar end by an analogue of N-acetylneuraminic acid. In particular, the methods of this invention employ xcex12,3-sialyltransferases to transfer a sialic acid or analogue thereof, activated as their CMP-nucleotide derivatives, to the non-reducing terminus of oligosaccharide acceptors.
Accordingly, in one of its method aspects, the present invention is directed to a method for the enzymatic synthesis of an xcex1-sialylated and fucosylated oligosaccharide containing a sialic acid or analogue thereof which method comprises the steps of:
a) selecting a sialyltransferase compatible with a fucosylated oligosaccharide having a fucose group in the non-reducing penultimate saccharide position;
b) selecting a CMP-sialic acid or an analogue thereof which is compatible with the sialyltransferase selected in step (a);
c) contacting said CMP-sialic acid or an analogue thereof with a fucosylated oligosaccharide of the formula 
wherein R1 represents a saccharide residue, R2 represents a saccharide residue, and R1 and R2 together represent an acceptor for the selected sialyltransferase; n is from 0 to about 10, Y is selected from the group consisting of O, NH and S, and R3 is selected from the group consisting of H, a protein, a lipid or an aglycon moiety having at least one carbon atom, in the presence of the sialyltransferase selected in step (a) above under conditions whereby the sialic acid or analogue thereof is transferred from the CMP-sialic acid or analogue thereof to the non-reducing sugar terminus of the fucosylated oligosaccharide so as to form an xcex1-sialylated fucosylated oligosaccharide containing a sialic acid or analogue thereof.
This invention is also directed to a method for the enzymatic synthesis of a fucosylated and xcex1-sialylated oligosaccharide which method comprises the steps of:
a) selecting a sialyltransferase capable of sialylating an oligosaccharide having a fucose group in the non-reducing penultimate saccharide position;
b) selecting a fucosyltransferase;
c) selecting a CMP-sialic acid or an analogue thereof which is compatible with the sialyltransferase selected in step (a);
d) selecting a GDP-fucose or an analogue thereof which is compatible with the fucosyltransferase selected in step (b);
e) contacting said CMP-sialic acid or an analogue thereof and said GDP-fucose or an analogue thereof with an oligosaccharide of the formula
xe2x80x83R1xe2x80x94R2-(saccharide)n-Yxe2x80x94R3
wherein R1 represents a saccharide residue, R2 represents a saccharide residue, and R1 and R2 together represent an acceptor for the selected sialyltransferase and the selected fucosyltransferase; n is from 0 to about 10, Y is selected from the group consisting of O, NH and S, and R3 is selected from the group consisting of H, a protein, a lipid or an aglycon moiety having at least one carbon atom, in the presence of said sialyltransferase and said fucosyltransferase selected in (a) and (b) above, under conditions whereby the sialic acid or analogue thereof and the fucose or analogue thereof are transferred from the CMP-sialic acid or analogue thereof and the GDP-fucose or analogue thereof, respectively, to the non-reducing sugar terminus of the oligosaccharide so as to form an xcex1-sialylated fucosylated oligosaccharide.
This invention is also directed to a method for determining the non-reducing terminus structure of an unknown oligosaccharide acceptor, which method comprises the steps of:
a) contacting the oligosaccharide acceptor with a sialyltransferase which is not capable of sialylating a non-reducing terminus of an oligosacchairde having a fucose group in the non-reducing penultimate saccharide position and determining whether the oligosaccharide was sialylated;
b) contacting the oligosaccharide with a sialyltransferase which is capable of sialylating a non-reducing terminus of an oligosacchairde having a fucose group in the non-reducing penultimate saccharide position and determining whether the oligosaccharide was sialylated; and
c) comparing the results to determine whether the non-reducing terminus was fucosylated. If the first aliquot of the unknown acceptor was not sialylated in step (a) and the second aliquot of acceptor was sialylated in step (b) then the acceptor was fucosylated in the non-reducing penultimate saccharide position.
The present invention is directed to the discovery that certain xcex12,3-sialyltransferases will transfer compatible analogues of sialic acid to certain oligosaccharides, glycoproteins, and glycolipids having a fucosyl group in the penultimate position to the non-reducing end of the sugar. This discovery permits the synthesis of oligosaccharides xcex1-sialylated at the non-reducing terminus from oligosaccharides having a fucosyl group in the penultimate position to the non-reducing end of the sugar. This method also permits the transfer of compatible analogues of sialic acid to the fucosylated oligosaccharide. This invention also permits the determination of the structure of acceptors and other qualitative and quantitive research of glycoproteins and glycolipids.
However, prior to discussing this invention in further detail, the following terms will first be defined.
A. Definitions
As used herein, the following terms have the definitions given below:
The term xe2x80x9csialic acidxe2x80x9d means all of the naturally occurring structures of sialic acid including 5-acetoamido-3,5-dideoxy-D-glycero-D-galacto-nonulopyranosylonic acid (xe2x80x9cNeu5Acxe2x80x9d) and the naturally occurring analogues of Neu5Ac, including N-glycolyl neuraminic acid (Neu5Gc) and 9-O-acetyl neuraminic acid (Neu5,9Ac2), which are compatible with the selected sialyltransferase. A complete list of naturally occurring sialic acids known to date are provided by Schauer31.
Naturally occurring sialic acids which are recognized by a particular xcex12,3-sialyltransferase so as to bind to the enzyme and are then available for transfer to an appropriate acceptor oligosaccharide structure are said to be compatible with the sialyltransferase and are sometimes referred to herein as a xe2x80x9ccompatible naturally occurring sialic acidxe2x80x9d.
The term xe2x80x9canalogues of sialic acidxe2x80x9d refers to analogues of naturally occurring structures of sialic acid including those wherein the sialic acid unit has been chemically modified so as to introduce, modify and/or remove one or more functionalities from such structures. For example, such modification can result in the removal of an OH functionality, the introduction of an amine functionality, the introduction of a halo functionality, and the like. In so far as the sialic acid analogues are compatible with the sialyltransferase, they are sometimes referred to herein as a xe2x80x9ccompatible sialic acid analoguesxe2x80x9d.
Certain analogues of sialic acid are known in the art and include, by way of example, 9-azido-Neu5Ac, 9-amino-Neu5Ac, 9-deoxy-Neu5Ac, 9-fluoro-Neu5Ac, 9-bromo-Neu5Ac, 8-deoxy-Neu5Ac, 8-epi-Neu5Ac, 7-deoxy-Neu5Ac, 7-epi-Neu5Ac, 7,8-bis-epi-Neu5Ac, 4-O-methyl-Neu5Ac, 4-N-acetyl-Neu5Ac, 4,7-di-deoxy-Neu5Ac, 4-oxo-Neu5Ac, 3-hydroxy-Neu5Ac, 3-fluoro-Neu5Ac acid as well as the 6-thio analogues of Neu5Ac. The nomenclature employed herein in describing analogues of sialic acid is as set forth by Reuter et al.20 
Insofar as sialyltransferases are designed to transfer or donate compatible naturally occurring sialic acids, analogues of Neu5Ac are sometimes referred to herein as xe2x80x9cartificial donorsxe2x80x9d whereas the compatible naturally occurring sialic acids are sometimes referred to herein as the xe2x80x9cnatural donorsxe2x80x9d.
The term xe2x80x9csialyltransferasexe2x80x9d refers to those enzymes which transfer a compatible naturally occurring sialic acid, activated as its cytidine monophosphate (CMP) derivative, to the terminal oligosaccharide structures of glycolipids or glycoproteins (collectively glycoconjugates) and include enzymes produced from microorganisms genetically modified so as to incorporate and express all or part of the sialyltransferase gene obtained from another source, including mammalian sources. Numerous sialyltransferases have been identified in the literature with the different sialyltransferases generally being distinguished from each other by the terminal saccharide units on the glycoconjugates which accept the transferase.64 For example, sialyltransferases, which build the following terminal oligosaccharide structures on glycoconjugates have been characterized:
xcex1Neu5Ac(2-3)xcex2Gal(1xe2x86x923/4)xcex2GlcNAc-21 
xcex1Neu5Ac(2-6)xcex2Gal(1-4)xcex2GlcNAc-21, 22 
xcex1Neu5Ac(2-3)xcex2Gal(1-3)xcex1GalNAc-23-25
xcex1Neu5Ac(2-6)xcex1GalNAc-26-28
xcex1Neu5Ac(2-6)xcex2GlcNAc-29, 30.
Other sialyltransferases with a variety of specificities have been isolated from a variety of sources.
A xe2x80x9csialyltransferase compatible with a fucosylated oligosaccharide having a fucose group in the non-reducing penultimate saccharide positionxe2x80x9d means that the sialyltransferase is capable of sialylating an oligosaccharide having a fucose group or analogue thereof in the non-reducing penultimate saccharide position. It has been found that the myxoma virus xcex12,3-sialyltransferase, as disclosed in International Patent Application Publication No. WO97/1830265, has this capability.
It is contemplated that related sialyltransferases also encoded by other genera of the sub-families of Chorodopoxvirinae, Entomopoxvirinae and the unclassified viruses of the family of Poxviridae will be compatible with a fucosylated oligosaccharide.
Analogues of sialic acid activated as their cytidinemonophosphate derivative which are recognized by a particular sialyltransferase so as to bind to the enzyme and are then available for transfer to an appropriate acceptor oligosaccharide structure are said to be compatible with the sialyltransferase and are sometimes referred to herein as a xe2x80x9ccompatible analogue of sialic acidxe2x80x9d. Because the transfer reaction employs a sialyltransferase, it goes without saying that an analogue of sialic acid employed in such a reaction must be a compatible analogue of sialic acid.
CMP-nucleotide derivative of Neu5Ac refers to the compound: 
CMP-derivatives of analogues of sialic acid refer to those compounds having structures similar to that above with the exception that the Neu5Ac residue is replace with an analogue of sialic acid.
The term xe2x80x9cfucosyltransferasexe2x80x9d refers to those enzymes which transfer a compatible naturally occurring fucose, activated as its guanosine diphosphate (GDP) derivative, to the terminal oligosaccharide structures of glycolipids or glycoproteins (collectively glycoconjugates) and include enzymes produced from microorganisms genetically modified so as to incorporate and express all or part of the fucosyltransferase gene obtained from another source, including mammalian sources. Numerous fucosyltransferases have been identified in the literature.
The term xe2x80x9canalogues of fucosexe2x80x9d refers to analogues of naturally occurring structures of fucose including those wherein the fucose unit has been chemically modified so as to introduce, modify and/or remove one or more functionalities from such structures. For example, such modification can result in the removal of an OH functionality, the introduction of an amine functionality, the introduction of a halo functionality, the introduction of a sulfate or phosphate moiety, and the like. Certain analogues of fucose are known in the art and include, by way of example, 3-deoxy-fucose68, arabinose, C-6 modified fucoses69 (i.e. 6-O-propyl fucose) and 3,6 dideoxy-L-galactose70.
It is also contemplated that the fucose or analogues of fucose may be transferred from other purine diphosphates including, adenosine-5xe2x80x2-diphospho-fucose, xanthosine-5xe2x80x2-diphospho-fucose, inosine-5xe2x80x2-diphospho-fucose, etc.71 
Analogues of fucose activated as their diphosphate derivative which are recognized by a particular fucosyltransferase so as to bind to the enzyme are then available for transfer to an appropriate acceptor oligosaccharide structure are said to be compatible with the fucosyltransferase. In so far as the fucose analogues are compatible with the fucosyltransferase, they are sometimes referred to herein as a xe2x80x9ccompatible fucose analoguesxe2x80x9d.
The term xe2x80x9coligosaccharidexe2x80x9d refers to compounds of the formula
xe2x80x83R1xe2x80x94R2-(saccharide)n-Yxe2x80x94R3
wherein R1 represents a saccharide residue, R2 represents a saccharide residue, and R1 and R2 together represent an acceptor for the selected sialyltransferase and the selected fucosyltransferase; n is from 0 to about 10, Y is selected from the group consisting of O, NH and S, and R3 is selected from the group consisting of H, a protein, a lipid or an aglycon moiety having at least one carbon atom.
The term xe2x80x9cfucosylated oligosaccharidexe2x80x9d refers to compounds of the formula 
wherein R1 represents a saccharide residue, R2 represents a saccharide residue, and R1 and R2 together represent an acceptor for the selected sialyltransferase and the selected fucosyltransferase; n is from 0 to about 10, Y is selected from the group consisting of O, NH and S, and R3 is selected from the group consisting of H, a protein, a lipid or an aglycon moiety having at least one carbon atom.
Since naturally occurring oligosaccharides and fucosylated oligosaccharides are acceptors for certain xcex12,3-sialyltransferases, and are believed to be acceptors of certain sialyltransferases in vivo, these oligosaccharides and fucosylated oligosaccharides are sometimes referred to herein as xe2x80x9cnatural acceptorsxe2x80x9d. Contrarily, since the oligosaccharides and fucosylated oligosaccharides employed in this invention are sometimes different from such xe2x80x9cnatural acceptorsxe2x80x9d, they are sometimes referred to herein as xe2x80x9cartificial acceptorsxe2x80x9d. That is to say that artificial acceptors are those oligosaccharides and fucosylated oligosaccharides which contain a substituent at the anomeric carbon atom of the reducing sugar which substituent is other than hydroxyl, a protein, or a lipid capable of forming a micelle or other large molecular weight aggregate. Accordingly, a protein linked to the anomeric carbon atom of the reducing sugar of the oligosaccharide or fucosylated oligosaccharide through its aglycon moiety would be an artificial acceptor since this acceptor contains an xe2x80x9cartificialxe2x80x9d unit, i.e., the aglycon linking group.
The fucosylated oligosaccharides of this invention may be further distinguished from natural acceptors by virtue of chemical modification(s) to one or more of the saccharide units of the oligosaccharide. Such chemical modification could involve the introduction and/or removal of one or more functionalities in one or more of the saccharide unit(s). For example, such modification can result in the removal of an OH functionality, the removal of saccharide unit(s), the introduction of an amine functionality, the introduction of a halo functionality, the introduction of one or more saccharide unit(s), and the like.
In a preferred embodiment, the aglycon moiety has from 1-20 carbon atoms and, more preferably, is selected from the group consisting of xe2x80x94(A)xe2x80x94Zxe2x80x2 wherein A represents a bond, an alkylene group of from 2 to 10 carbon atoms, and a moiety of the form xe2x80x94(CH2CR4R5G)n(CH2CR4R5)xe2x80x94 wherein n is an integer equal to 0 to 5; R4 and R5 are independently selected from the group consisting of hydrogen, phenyl, phenyl substituted with 1 to 3 substituents selected from the group consisting of amine, hydroxyl, halogen, alkyl of from 1 to 4 carbon atoms and alkoxy of from 1 to 4 carbon atoms, methyl, or ethyl; and G is selected from the group consisting of a bond, oxygen, sulphur, NH, and Zxe2x80x2 is selected from the group consisting of hydrogen, methyl, xe2x80x94OH, xe2x80x94SH, xe2x80x94NH2, xe2x80x94NHR6, xe2x80x94N(R6)2, xe2x80x94C(O)OH, xe2x80x94C(O)OR6, xe2x80x94C(O)NHNH2, xe2x80x94C(O)NH2, xe2x80x94C(O)NHR6, xe2x80x94C(O)N(R6)2, and xe2x80x94OR7 wherein each R6 is independently alkyl of from 1 to 4 carbon atoms and R7 is an alkenyl group of from 3 to 10 carbon atoms. Preferably, the xe2x80x94(A)xe2x80x94Zxe2x80x2 group defines a group capable of being linked to a carrier or a group capable of being derivatized to a group which is capable of being linked to a carrier.
Preferably, the aglycon group is a hydrophobic group of at least 2 carbon atoms and more preferably at least 4 carbon atoms. Most preferably the aglycon group is xe2x80x94(CH2)8COOMe.
When the aglycon group is one which is capable of being linked to a carrier such as an antigenic carrier, the methods of this invention are useful in preparing artificial conjugates such as artificial antigens having one or more xcex1-sialylated oligosaccharide groups containing an analogue of sialic acid which groups are pendant to the antigen.
The carrier is a low or high molecular weight, nonimmunogenic or antigenic carrier including the linking to a fluorescent label, a radioactive label, biotin, or a photolabile linking arm or a moiety to be targeted. Preferably, the carrier is an antigenic carrier and accordingly, the artificial conjugate is an artificial antigen. In some cases it may be advantageous to employ a non-immunogenic carrier.
On the other hand, the carrier can be a low molecular weight carrier such as ethylene diamine, hexamethylene diamine, tris(2-aminoethyl)amine, L lysilysine, poly-L-lysine, and polymers of various molecular weights.
Saccharide units (i.e., sugars) useful in the oligosaccharides described above include by way of example, all natural and synthetic derivatives of glucose, galactose, N-acetyl-glucosamine, N-acetyl-galactosamine, fucose, sialic acid, 3-deoxy-D,L-octulosonic acid and the like. In addition to being in their pyranose form, all saccharide units in the oligosaccharides are in their D form except for fucose which is in its L form.
As noted above, oligosaccharides useful in the processes disclosed herein contain terminal units which are compatible with the selected sialyltransferase. That is to say that such compatible terminal units permit recognition of the oligosaccharide by a particular sialyltransferase so that the sialyltransferase binds to the oligosaccharide and further permits transfer of the compatible analogue of sialic acid onto the oligosaccharide.
B. Synthesis and Methodology
Preparation of Oligosaccharides
Oligosaccharides to which the sialic acid analogue is to be enzymatically coupled are readily prepared either by complete chemical synthesis or by chemical/enzymatic synthesis wherein glycosyltransferases (other than sialyltransferases) are employed to effect the sequential addition of one or more sugar units onto a saccharide or an oligosaccharide. Such methods are well known in the art. For example, chemical synthesis is a convenient method for preparing either the complete oligosaccharide glycoside; for chemically modifying a saccharide unit which can then be chemically or enzymatically coupled to an oligosaccharide glycoside; or for chemically preparing an oligosaccharide glycoside to which can be enzymatically coupled one or more saccharide units.
Chemical modifications of saccharide units are well known in the art which methods are generally adapted and optimized for each individual structure to be synthesized. In general, the chemical synthesis of all or part of the oligosaccharide first involves formation of a glycosidic linkage on the anomeric carbon atom of the reducing sugar. Specifically, an appropriately protected form of a naturally occurring or of a chemically modified saccharide structure (the glycosyl donor) is selectively modified at the anomeric center of the reducing unit so as to introduce a leaving group comprising halides, trichloroacetimidate, thioglycoside, etc. The donor is then reacted under catalytic conditions (e.g., a soluble silver salt such as silver trifluoromethanesulfonate, a Lewis acid such as boron trifluoride etherate or trimethylsilyltrifluoromethanesulfonate, or thioglycoside promoters such as methyl trifluoromethanesulfonate or dimethyl(methylthio)sulfonium trifluoromethanesulfonate) with an aglycon or an appropriate form of a carbohydrate acceptor which possess one free hydroxyl group at the position where the glycosidic linkage is to be established. A large variety of aglycon moieties are known in the art and can be attached with the proper configuration to the anomeric center of the reducing unit. Appropriate use of compatible blocking groups, well known in the art of carbohydrate synthesis, will allow selective modification of the synthesized structures or the further attachment of additional sugar units or sugar blocks to the acceptor structures.
After formation of the glycosidic linkage, the oligosaccharide can be used to effect coupling of additional saccharide unit(s) or chemically modified at selected positions or, after conventional deprotection, used in an enzymatic synthesis. In general, chemical coupling of a naturally occurring or chemically modified saccharide unit to the saccharide glycoside is accomplished by employing established chemistry well documented in the literature. See, for example, Okamoto et al.32, Ratcliffe et al.33, Abbas et al.34, Paulsen35, Schmidt36, Fugedi et al.37, and Kameyama et al..38. The disclosures of each of these references are incorporated herein by reference in their entirety.
On the other hand, enzymatic coupling is accomplished by the use of glycosyl transferases which transfer sugar units, activated as their appropriate nucleotide donors, to specific saccharide or oligosaccharide acceptors, generally at the non-reducing sugar portion of the saccharide or oligosaccharide. See, for example, Toone et al.62 and U.S. Pat. No. 5,374,65567. Moreover, it is possible to effect selected chemical modifications of the saccharide or oligosaccharide acceptor, of the sugar donor or the product of the enzymatic reaction so as to introduce modifications or further modifications into the structure.
Preparation of Analogues of Sialic Acid
Certain analogues of sialic acid are well known in the art and are prepared by chemical modification of sialic acid using procedures well documented in the art. For example, chemically modified Neu5Ac derivatives including 9-azido-Neu5Ac.39, various 9-amino-Neu5Ac derivatives40, 9-deoxy-Neu5Ac41, 9-fluoro-Neu5Ac42, 9-bromo-Neu5Ac43, 8-deoxy-Neu5Ac41, 8-epi-Neu5Ac44, 7-deoxy-Neu5Ac47-epi-Neu5Ac45, 7,8-bis-epi-Neu5Ac.45, 4-O-methyl-Neu5Ac53, 4-N-acetyl-Neu5Ac48, 4-epi-Neu5Ac47, 4,7-di-deoxy-Neu5Ac41, 4-oxo-Neu5Ac49, 4-deoxy-Neu5Ac52, 3-hydroxy-Neu5Ac50, 3-fluoro-Neu5Ac51 acid, the product of cleavage of the side chain at C-8 or at C-746 as well as the 6-thio analogues of Neu5Ac54 are reported in the literature. Other sialic acid analogues are disclosed in U.S. Pat. No. 5,352,6703. Chemical modification leading to other sialic acid analogues would follow such established procedures.
Activation of Analogues of Sialic Acid to Their CMPxe2x80x94Nucleotide Derivatives
The enzymatic transfer of analogues of sialic acid require the prior synthesis (i.e., activation) of their nucleotide (CMP) derivatives. Activation of the analogues of sialic acid is usually done by using the enzyme CMP-sialic acid synthase which is readily available and the literature provides examples of the activation of various analogues of sialic acid such as 9-substituted Neu5Ac28,39,40,55-57, 7-epiNeu5Ac58, 7,8-bis-epi-Neu5Ac58, 4-O-methyl-Neu5Ac59, 4-deoxy-Neu5Ac60, 4-acetamido-Neu5Ac48, 7-deoxy-Neu5Ac56, 4,7-dideoxy-Neu5Ac56, the 6-thio derivatives of Neu5Ac61 and Neu5OH (KDN). Still other examples of activated sialic acid analogues are disclosed in U.S. Pat. No. 5,352,6703.
Transfer of the Analogues of Sialic Acid to the Oligosaccharide Acceptor
The nucleotide derivative of a compatible analogue of sialic acid and the compatible acceptor (i.e., a fucosylated oligosaccharide or an oligosaccharide having terminal saccharide unit(s) on the non-reducing end which are recognized by the selected sialyltransferase) are combined with each other in the presence of the selected sialyltransferase compatible with a fucosylated oligosaccharide under conditions wherein the sialic acid or analogue thereof is transferred to the acceptor. As is apparent, the saccharide or oligosaccharide acceptor employed must be one which functions as a substrate of the particular sialyltransferase employed.
In this regard, the art recognizes that artificial acceptors are tolerated in some cases by sialyltransferases especially where modification is in the aglycon part of the structure.
Likewise, when an analogue of sialic acid (i.e., an artificial donor) is to be enzymatically transferred, it is necessary that the CMP derivative of the analogue also be recognized by the sialyltransferase. In this regard, the art recognizes that certain sialyltransferases can tolerate some modifications to naturally occurring sialic acids and still transfer these analogues of sialic acid to glycoproteins or glycolipids possessing a suitable terminal acceptor structure.
It has been found that sialyltransferases possess sufficient recognition flexibility so as to transfer an artificial donor to an artificial acceptor3. Such flexibility permits the facile synthesis of a panel of oligosaccharides containing different analogues of sialic acid at the non-reducing sugar terminus of the oligosaccharide.
As noted above, a nucleotide derivative of a compatible sialic acid or a compatible analogue thereof is combined with a compatible acceptor (i.e., a saccharide or an oligosaccharide having terminal saccharide unit(s) on the nonreducing end which are recognized by the selected sialyltransferase) in the presence of the sialyltransferase under conditions wherein the sialic acid or analogue thereof is transferred to the acceptor. Suitable conditions, known in the art, include the addition of the appropriate sialyltransferase to a mixture of the compatible acceptor and of the CMP-derivative of the compatible sialic acid analogue in a appropriate buffer such as 0.1M sodium cacodylate in appropriate conditions of pH and temperature such as at a pH of 6.5 to 7.5 and a temperature between 25xc2x0 and 45xc2x0 C., preferably 35-40xc2x0 C. for 12 hours to 4 days. The resulting oligosaccharide can be isolated and purified using conventional methodology comprising HPLC, ion exchange-, gel-, reverse-phase- or adsorption chromatography.
Once formed, the xcex1-sialylated oligosaccharide glycoside can be further modified by chemical and/or enzymatic means to further derivatize this compound. For example, other glycosyltransferases can be used to add a glycosyl group to an xcex1-sialylated oligosaccharide recognized by the transferase. This latter aspect is important insofar as the modifications made to the oligosaccharide must be compatible with the desired enzymatic transfers.
Additionally, the xcex1 sialylated oligosaccharide can be chemically modified to provide further derivatization of these compounds. Such chemical modification includes reduction of a 9-azido group on an analogue of sialic acid to an amine group which can be still further functionalized to another derivative such as the 9-acetamido derivative. Similarly, the carboxyl group found on analogues of sialic acid can be selectively transformed on xcex1 sialylated oligosaccharide glycosides via lactonization, reduction or transformation into an amide.
In one or more of the enzymatic steps recited above, the enzyme can be bound to a solid support so as to facilitate the reaction of the reagents and the recovery of the product from the enzyme.
C. Utility
The methods of this invention are useful in preparing oligosaccharides containing sialic acid or an analogue thereof bound via an xcex1-linkage to the non-reducing sugar terminus of the oligosaccharide. Such oligosaccharides are recognized in the art as being useful as pharmaceuticals, as well as in the generation of antibodies to these structures, which antibodies are useful in diagnostic assays.
Additionally, methods of this invention are useful in preparing oligosaccharides containing an analogue of sialic acid bound via an xcex1-linkage to the non-reducing sugar terminus of the oligosaccharide which can be coupled to an antigenic carrier so as to produce artificial antigens. Accordingly, such oligosaccharides act as intermediates in the preparation of artificial antigens.
Additionally, methods of this invention are useful in the determination of the non-reducing terminus of an unknown oligosaccharide. Specifically, a first aliquot of the unknown oligosaccharide acceptor can be contacted with a sialyltransferase which is not capable of sialylating a non-reducing terminus of an oligosaccharide having a fucose group in the non-reducing penultimate saccharide position and determining whether the oligosaccharide was sialylated; a second aliquot of the unknown oligosaccharide is also contected with an xcex1-2,3 sialyltransferase which is capable of sialylating a non-reducing terminus of an oligosacchairde having a fucose group in the non-reducing penultimate saccharide position and determining whether the oligosaccharide was sialylated; the results are compared to determine whether the non-reducing terminus was fucosylated. If the first aliquot of the unknown acceptor was not sialylated and the second aliquot of acceptor was sialylated then the acceptor was fucosylated in the non-reducing penultimate saccharide position.